1. Field of the Invention
The present invention relates to a solenoid-type hammer assembly for an impact printer and, more particularly, to an improved design of an electromagnetically actuated hammer mechanism of the type used to strike the printing element in an impact printer.
2. Description of the Prior Art
The advent of high speed data processing and word processing equipment has brought about a need for high speed printers capable of producing high quality printing. Perhaps the most popular type of printer in such applications is that which uses the so-called "daisy wheel". The daisy wheel basically consists of a disc having a plurality of flexible fingers or petals around its periphery, each of which bears a particular print character. In operation, a daisy wheel is rotated until the selected character is in position for printing, at which time a hammer means drives the print element against the paper to be printed. A typical example of such a printer is disclosed in U.S. Pat. No. 3,954,163 to Gabor.
In such printers, it is necessary to have a significant clearance space between the hammer and the print wheel or daisy wheel when the hammer is withdrawn so that the print wheel may rotate freely without danger of striking the hammer and damaging the relatively fragile spokes or petals of the daisy wheel. Accordingly, the hammer must have a relatively long stroke so that it can be withdrawn a significant distance away from the print wheel as the print wheel rotates, while being capable of driving the printing element against the paper to be printed when the print wheel has stopped. The obvious approach to achieving such a long hammer stroke would be to propel the hammer by a strong magnetic field produced by a relatively large solenoid. However, in such printers, in order to achieve high speed, it is desirable to keep the hammer assembly as light in weight as possible, thereby effectively precluding the use of a large, heavy solenoid.
In order to make use of relatively small, lightweight electromagnetic coils, the initial prior art daisy wheel printers used the coils to drive a relatively long clapper arm, much like that used in an electric doorbell. The end of the clapper arm traveled in a relatively wide arch to strike the hammer, thereby allowing the hammer to have the necessary long stroke while using coils which are relatively small and lightweight. Such a hammer assembly soon proved to be undesirable, primarily because the driving of the hammer indirectly through a second drive element, i.e. the clapper, limits the ability to control the impact force of the hammer against the print element.
In order to overcome these objections, an entirely new type of hammer assembly was developed, one with a specially designed pole piece and armature. This type of hammer assembly is disclosed in U.S. Pat. No. 4,239,401 to Veale. In the hammer assembly of Veale, the armature is formed with a tapered leading surface which is essentially in the shape of a truncated right frusticone. The pole piece is formed with a recess which conforms to the shape of the armature's leading surface. This configuration of the armature and pole piece provides for a magnetic force field having essentially parallel lines of force between the armature and the pole piece so that the magnetic force between the armature and pole piece is substantially constant as the armature moves toward the pole piece. The advantages of such a force field are two fold: (1) the force on the armature being constant, regardless of the distance from the pole piece, the acceleration of the armature is constant; and (2) since the lines of force do not diverge as the distance from the pole piece increases, the initial position of the armature can be a significant distance from the pole piece and still be within an area of substantial magnetic field strength, thereby allowing the armature to be propelled toward the pole piece for a relatively long distance. This configuration thus allows the armature to be made unitary with the hammer so that the hammer can travel with the necessary length of stroke without the need of a clapper. Not only is the mechanism of the Veale hammer assembly greatly simplified over that of the prior art hammer/clapper assembly, but elimination of the limitations inherent in the use of the clapper allow a capability of much higher printing speed while providing for greater control of the hammer's actions.
The hammer assembly of the Veale patent is provided with a polyurethane cushion against which the armature strikes after the hammer rebounds from the impact against the printing element. This cushion absorbs a significant part of the momentum of the armature/hammer element, thereby attempting to bring the armature/hammer element to a complete stop almost immediately after impact against the printing element.
While the Veale-type hammer assembly represents a considerable improvement over the hammer/clapper type of hammer assembly, significant problems were encountered in use. That is, the Veale hammer assembly was found to have an insufficient stroke which would lead to the ram striking out by bottoming out in the pole piece. An increased stroke is required when the daisy wheel breaks and the hammer must reach the platen before the ram bottoms out. It was also found that the polyurethane cushion alone did not absorb substantially all of the momentum of the armature/hammer element so that there has been excessive hammer settling time which limits the printer speed. Along the same lines, there was found to be excessive hammer rebound from the cushion which could cause the hammer to strike the rotating wheel, damaging the hammer and the wheel. Also encountered was an inconsistency in impact energy due to random friction and a relatively short life expectancy because of wear of moving parts.
In the Veale hammer assembly, spring tangs at the end of a spring positioned in tang holes were used to prevent rotation of the hammer assembly. However, it has been found that this is ineffectual in preventing hammer rotation over a long period of time, the result being that the hammer rotates and strikes two adjacent print elements.
Another area of significant concern is that the hammer assembly of Veale is manufactured in an expensive manner, because of the large number of parts, resulting in a product having an excessive cost and an inadequate life expectancy. Problems were also encountered in assembling and aligning the multiple parts.